In addition to when, the big unknown is what Biden's decision will be: Is he in our is he out?

See photos of Biden as he mulls over a presidential run:

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Joe Biden as he mulls over a presidential run

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Biden nears decision time

NEW YORK - SEPTEMBER 10: Stephen talks with Vice President Joe Biden, on The Late Show with Stephen Colbert, Thursday Sept 10, 2015 on the CBS Television Network. (Photo by Jeffrey R. Staab/CBS via Getty Images)

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Biden's public schedule gives little indication of his decision. He traveled from his home in Delaware to Washington Monday morning where he will address a climate change summit this afternoon.